Simple hydrocarbon. Refers to aromatic hydrocarbons, a class of organic substances.
The substance is a clear liquid, colorless, has a sweetish characteristic odor. Benzene is classified as an unsaturated hydrocarbon. The famous formula for the benzene ring was proposed by the Nobel laureate in chemistry, Linus Pauling. It was he who proposed to depict benzene in the form of a hexagon with a circle inside. This image gives an understanding of the absence of double bonds and the presence of a single electron cloud, which covers all 6 carbon atoms.
Formula
Getting benzene
Natural Sources
The natural source for obtaining benzene is coal. The process of coking coal was discovered by Michael Faraday back in 1825. He studied the lighting gas used in street lamps and was able to isolate and describe benzene. Now practically no benzene is obtained from coal tar in this way. There are many other more productive ways to get it.
Artificial sources of obtaining
- Artificial catalytic reforming of gasoline. Gasoline oil fractions are used for production. In this process, a large amount of toluene is formed. The market demand for toluene is not very high, so benzene is also further produced from it. Benzene is obtained from heavy fractions of oil by pyrolysis through the process of dealkylation of a mixture of toluene, xylene.
- Obtaining by the Reppe method. Until 1948, benzene was obtained using the Berthelot method by passing acetylene over activated carbon at a temperature of 400°C. The yield of benzene was large, but a multicomponent mixture of substances was obtained, which was difficult to purify. In 1948, Reppe replaced activated carbon with nickel. The result was benzene. The process is called acetelene trimerization- three molecules of acetylene are converted into one benzene:
3C 2 H 2 → C 6 H 6 .
properties of benzene
Physical properties
During combustion, a large amount of soot is released, since the hydrocarbon is unsaturated (it lacks 8 hydrogen atoms to meet the standard formula of saturated hydrocarbons). At low temperatures, benzene becomes a white crystalline mass.
Chemical properties
Benzene enters into substitution reactions in the presence of catalysts - usually Al(3+) or Fe(3+) salts:
- Halogenation - a qualitative reaction to benzene with Br 2:
C 6 H 6 + Br 2 \u003d C 6 H 5 Br + HBr.
- Nitration - interaction with nitric acid. In organic chemistry, this process is accompanied by the elimination of the OH group:
C 6 H 6 + HO-NO 2 → C 6 H 5 NO 2 + H 2 O.
- Catalytic alkylation leads to the production of benzene homologues - alkylbenzenes:
C 6 H 6 + C 2 H 5 Cl → C 6 H 5 C 2 H 5 + HCl.
Benzene homologues containing a radical react differently than benzene itself. Reactions go differently and often in the light:
- halogenation C 6 H 5 -CH 3 + Br 2 (in the light) = C 6 H 5 -CH 2 Br + HBr;
- nitration - C 6 H 5 -CH 3 + 3HNO 3 → C 6 H 2 CH 3 (NO 2) 3.
Benzene oxidation reactions are very complex and are not typical for this substance. Oxidation is characteristic of homologues. Here, for example, the reaction for obtaining benzoic acid:
C 6 H 5 CH 3 + [O] → C 6 H 5 COOH.
The combustion process of a substance occurs according to the standard scheme for all organic substances:
C n H 2n-6 + (3n-3) \ 2 O 2 → nCO 2 + (n-3) H 2 O.
Hydrogenation reactions. The reaction is difficult, catalysts, pressure, temperature are required. In the reactions of benzene with hydrogen, cyclohexane is obtained:
C 6 H 6 + 3H 2 → C 6 H 12.
And in reactions with alkylbenzene - methylcyclohexane, where one hydrogen atom is replaced by a radical group -CH 3:
C 6 H 5 CH 3 + 3H 2 → C 6 H 11 -CH 3.
Application of benzene
Benzene in its pure form is practically not used. It is produced for the production of other important compounds, such as, for example, ethylbenzene, from which styrene and polystyrene are obtained.
The lion's share of benzene is used for the production of phenol, which is necessary in the production of nylon, dyes, pesticides, and drugs. The famous drug aspirin cannot be obtained without the participation of phenol.
Cyclohexane from benzene is necessary for the production of plastics and artificial fibers, nitrobenzene goes to the production of aniline, which is used to produce rubbers, dyes and herbicides.
The cyclic structure of benzene was first proposed by F.A. Kekule in 1865
Friedrich August Kekule von Stradonitz is an outstanding German chemist of the 19th century. In 1854, he discovered the first organic compound containing sulfur - thioacetic acid (thioethanoic acid). In addition, he established the structure of diazo compounds. However, his most famous contribution to the development of chemistry is the establishment of the structure of benzene (1866). Kekule showed that the double bonds of benzene alternate around the ring (this idea first occurred to him in a dream). He later showed that the two possible double bond arrangements are identical and that the benzene ring is a hybrid between the two structures. Thus, he anticipated the concept of resonance (mesomerism), which appeared in the theory of chemical bonding in the early 1930s.
If benzene really had such a structure, then its 1,2-disubstituted derivatives should have two isomers each. For example,
However, none of the 1,2-disubstituted benzenes can isolate two isomers.
Therefore, Kekule subsequently suggested that the benzene molecule exists as two structures rapidly passing into each other:
Note that such schematic representations of benzene molecules and their derivatives usually do not indicate the hydrogen atoms attached to the carbon atoms of the benzene ring.
In modern chemistry, the benzene molecule is considered as a resonant hybrid of these two limiting resonant forms (see Section 2.1). Another description of the benzene molecule is based on a consideration of its molecular orbitals. In sec. 3.1, it was indicated that the -electrons located in the -bonding orbitals are delocalized between all carbon atoms of the benzene ring and form an -electron cloud. In accordance with this representation, the benzene molecule can be conventionally depicted as follows:
Experimental data confirm the presence of just such a structure in benzene. If benzene had the structure that Kekule originally proposed, with three conjugated double bonds, then benzene would have to enter into addition reactions like alkenes. However, as mentioned above, benzene does not enter into addition reactions. In addition, benzene is more stable than if it had three isolated double bonds. In sec. 5.3 it was indicated that the enthalpy of hydrogenation of benzene with the formation of cyclohexane has a larger negative
Table 18.3. Length of various carbon-carbon bonds
Rice. 18.6. The geometric structure of the benzene molecule.
value than three times the enthalpy of hydrogenation of cyclohexene. The difference between these values is usually called the delocalization enthalpy, resonant energy, or benzene stabilization energy.
All carbon-carbon bonds in the benzene ring have the same length, which is less than the length of C-C bonds in alkanes, but longer than the length of C=C bonds in alkenes (Table 18.3). This confirms that the carbon-carbon bonds in benzene are a hybrid between single and double bonds.
The benzene molecule has a flat structure, which is shown in Fig. 18.6.
Physical properties
Under normal conditions, benzene is a colorless liquid that freezes at 5.5°C and boils at 80°C. It has a characteristic pleasant smell, but, as mentioned above, is highly toxic. Benzene is immiscible with water, and in the benzene system, water forms the top of the two layers. However, it is soluble in non-polar organic solvents and is itself a good solvent for other organic compounds.
Chemical properties
Although benzene enters into certain addition reactions (see below), it does not exhibit the reactivity typical of alkenes in them. For example, it does not decolorize bromine water or α-ion solutions. In addition, benzene
enters into addition reactions with strong acids, such as hydrochloric or sulfuric acid.
At the same time, benzene takes part in a number of electrophilic substitution reactions. Aromatic compounds are the products of reactions of this type, since the delocalized -electron system of benzene is preserved in these reactions. The general mechanism of substitution of a hydrogen atom on a benzene ring by some electrophile is described in Sec. 17.3. Examples of electrophilic substitution of benzene are its nitration, halogenation, sulfonation, and Friedel-Crafts reactions.
Nitration. Benzene can be nitrated (introducing a group into it) by treating it with a mixture of concentrated nitric and sulfuric acids:
Nitrobenzene
The conditions for this reaction and its mechanism are described in Sec. 17.3.
Nitrobenzene is a pale yellow liquid with a characteristic almond odor. During the nitration of benzene, in addition to nitrobenzene, crystals of 1,3-dinitrobenzene are also formed, which is the product of the following reaction:
Halogenation. If you mix benzene in the dark with chlorine or bromine, no cancer will occur. However, in the presence of catalysts with the properties of Lewis acids, electrophilic substitution reactions occur in such mixtures. Typical catalysts for these reactions are iron(III) bromide and aluminum chloride. The action of these catalysts is that they create polarization in the halogen molecules, which then form a complex with the catalyst:
although there is no direct evidence that free ions are formed in this case. The mechanism of benzene bromination using iron (III) bromide as an ion carrier can be represented as follows:
Sulfonation. Benzene can be sulfonated (replacing a hydrogen atom in it with a sulfo group) by refluxing its mixture with concentrated sulfuric acid for several hours. Instead, benzene can be gently heated mixed with fuming sulfuric acid. Fuming sulfuric acid contains sulfur trioxide. The mechanism of this reaction can be represented by the scheme
Friedel-Crafts reactions. Friedel-Crafts reactions were originally called condensation reactions between aromatic compounds and alkyl halides in the presence of an anhydrous aluminum chloride catalyst.
In condensation reactions, two molecules of reactants (or one reactant) combine with each other, forming a molecule of a new compound, while a molecule of some simple compound, such as water or hydrogen chloride, is split off (eliminates) from them.
Currently, the Friedel-Crafts reaction is any electrophilic substitution of an aromatic compound in which a carbocation or a highly polarized complex with a positively charged carbon atom plays the role of an electrophile. The electrophilic agent is usually an alkyl halide or chloride of a carboxylic acid, although it can also be, for example, an alkene or an alcohol. Anhydrous aluminum chloride is usually used as a catalyst for these reactions. Friedel-Crafts reactions are usually divided into two types: alkylation and acylation.
Alkylation. In Friedel-Crafts reactions of this type, one or more hydrogen atoms in the benzene ring are replaced by alkyl groups. For example, when a mixture of benzene and chloromethane is heated carefully in the presence of anhydrous aluminum chloride, methylbenzene is formed. Chloromethane plays the role of an electrophilic agent in this reaction. It is polarized by aluminum chloride in the same way as it happens with halogen molecules:
The mechanism of the reaction under consideration can be represented as follows:
It should be noted that in this condensation reaction between benzene and chloromethane, a hydrogen chloride molecule is split off. We also note that the real existence of a metal carbocation in the form of a free ion is doubtful.
Alkylation of benzene with chloromethane in the presence of a catalyst - anhydrous aluminum chloride does not end with the formation of methylbenzene. In this reaction, further alkylation of the benzene ring occurs, leading to the formation of 1,2-dimethylbenzene:
Acylation. In Friedel-Crafts reactions of this type, a hydrogen atom in the benzene ring is replaced by an acyl group, resulting in the formation of an aromatic ketone.
The acyl group has the general formula
The systematic name of an acyl compound is formed by replacing the suffix and ending -ova in the name of the corresponding carboxylic acid, of which the given acyl compound is a derivative, with the suffix -(o)yl. For example
Acylation of benzene is carried out using a chloride or anhydride of a carboxylic acid in the presence of an anhydrous aluminum chloride catalyst. For example
This reaction is a condensation in which the elimination of a hydrogen chloride molecule occurs. Note also that the name "phenyl" is often used to denote the benzene ring in compounds where benzene is not the main group:
Addition reactions. Although benzene is most characteristic of electrophilic substitution reactions, it also enters into some addition reactions. We have already met one of them. We are talking about the hydrogenation of benzene (see Section 5.3). When a mixture of benzene and hydrogen is passed over the surface of a finely ground nickel catalyst at a temperature of 150–160 °C, a whole sequence of reactions occurs, which ends with the formation of cyclohexane. The overall stoichiometric equation for this reaction can be represented as follows:
Under the influence of ultraviolet radiation or direct sunlight, benzene also reacts with chlorine. This reaction is carried out by a complex radical mechanism. Its final product is 1,2,3,4,5,6-hexachlorocyclohexane:
A similar reaction takes place between benzene and bromine under the action of ultraviolet radiation or sunlight.
Oxidation. Benzene and the benzene ring in other aromatic compounds are generally resistant to oxidation even by such strong oxidizing agents as an acidic or alkaline solution of potassium permanganate. However, benzene and other aromatics burn in air or oxygen to form a very smoky flame, which is typical for hydrocarbons with a high relative carbon content.
Benzene homologues are capable of reacting in two directions with the participation of an aromatic nucleus and a side chain (alkyl groups), depending on the nature of the reagent.
1. Reactions on the aromatic nucleus
Due to the donor effect of the alkyl group, the reactions S E ArH go to ortho- And pair- positions of the aromatic nucleus, while the conditions are milder than for benzene.
a) halogenation
b) nitration
Notice how, as the number of acceptor groups (-NO 2) increases, the temperature of the nitration reactions rises.
c) sulfonation
The reaction predominantly produces P-isomer.
d) alkylation
e) acylation
2. Side chain reactions
The alkyl fragment of the benzene molecule enters into reactions S R with the participation of a carbon atom in α -position (benzyl position).
Oxidation of all homologues of benzene KMnO 4 /100°C leads to the formation of benzoic acid.
condensed arenas
Condensed arenas are aromatic systems (n=2 and 3). The degree of aromaticity of condensed arenes is lower than for benzene. They are characterized by electrophilic substitution reactions, addition and oxidation reactions occurring under milder conditions than for benzene.
Reactivity of naphthalene
S E ArH reactions for naphthalene proceed mainly according to α -position, except for sulfonation. The electrophilic addition of Ad E proceeds at positions 1,4, while naphthalene exhibits the properties of conjugated dienes.
1. Electrophilic substitution reactions,S E ArH
2. Reactions of electrophilic addition, reduction and oxidation.
Reactivity of anthracene and phenanthrene
The reactions of electrophilic substitution, S E ArH and electrophilic addition, Ad E for anthracene proceed predominantly at positions 9 and 10 (see the scheme below).
The reactions of electrophilic substitution, S E ArH and electrophilic addition, Ad E for phenanthrene proceed predominantly at positions 9 and 10, as for anthracene (see the diagram below).
Oxidation and reduction reactions for anthracene and phenanthrene.
Structures of some drugs based on naphthalene, anthracene and phenanthrene
Naphthyzin(nafazolin, sanorin)
vasoconstrictor action(treatment of rhinitis, sinusitis)
(the parent structure is underlined in the title, pay attention to the numbering)
Naftifin
antifungal action (treatment of dermatitis)
Nabumeton
anti-inflammatory, antipyretic, analgesic action(treatment of osteoarthritis, rheumatoid arthritis).
Nadolol
(the term cis, in this case, denotes the mutual arrangement of hydroxyl groups)
hypotensive(lowers blood pressure) and antiarrhythmic action
Morphine, codeine
Security questions for the chapter "ARENA"
1. What properties of benzene distinguish it from other unsaturated compounds - alkenes, alkynes? What does the term "aromatic compound" mean?
2. Write the structural formulas of the compounds: a) ethylbenzene; b) 1,3-dimethylbenzene ( m -xylene); c) 1,3,5-trimethylbenzene (mesitylene); d) isopropylbenzene (cumene); e) 3-phenylpentane; f) vinylbenzene (styrene); g) phenylacetylene; h) trance -diphenylethylene ( trance -stilbene).
3. Describe the structural features of compounds that exhibit aromaticity. State Hückel's rule. Which of the following compounds are aromatic?
4. Compare the ratio of cyclohexene and benzene to the following reagents under the indicated conditions : a) Br 2 (H 2 O.20 C); b) KMnO 4 (H 2 Oh, 0 C); c) H 2 SO 4 (conc.), 20 C; d) H 2 (Pd, 30 C); before 3 , then H 2 O(Zn); e) HBr.
5. Write the structural formulas of monosubstituted benzene formed in the reactions of benzene with the following reagents: a) H 2 SO 4 (conc.); b) HNO 3 ; H 2 SO 4 (conc.); c) Br 2 /fe; d) Cl 2 /AlCl 3 ; e) CH 3 Br/AlBr 3 ; e) CH 3 COCl/AlCl 3 . Name the reactions and their products. Indicate with which electrophile benzene reacts in each specific case.
6. Give a general scheme for the interaction of benzene with an electrophilic reagent ( E + ). Name intermediate complexes. Which step usually determines the rate of a reaction? Give a graph of the change in the potential energy of the reaction under consideration.
7. Define the following concepts: a) transition state; b) intermediate connection; c) -complex; d) -complex. Which of them are identical? Illustrate these concepts using the example of benzene bromination in the presence of a catalyst. FeBr 3 .
8. Using the example of the reactions of ethene and benzene with bromine, compare the mechanism of electrophilic addition in alkenes with the mechanism of electrophilic substitution in the aromatic series. At what stage is the difference observed and why?
9. Using inductive and mesomeric effects, describe the interaction of the substituent with the benzene ring in the indicated compounds:
Note the electron-donating (ED) and electron-withdrawing (EA) substituents.
10. Write the mononitration schemes for the following compounds: a) phenol; b) benzenesulfonic acids; c) isopropylbenzene; d) chlorobenzene. For which compound should the relative substitution rate be the highest and why?
11. The formation of what products should be expected during monosulfonation of compounds: a) toluene; b) nitrobenzene; c) benzoic acid; d) bromobenzene? Which compound should be sulfonated the easiest? Why?
12. Arrange the following compounds in a row according to the increase in reactivity when they are brominated into a benzene ring: a) benzene; b) phenol; c) benzaldehyde; d) ethylbenzene. Give explanations.
13. Name the following hydrocarbons:
14. Write the reactions of benzene with the following reagents : a) Cl 2 (Fe); b) 3Cl 2 (light); c) HNO 3 (H 2 SO 4 ); d) Oh 2 (air) (V 2 ABOUT 5 , 450 C); e) 3O 3 , then H 2 O(Zn); f) H 2 SO 4 (oleum); g) 3H 2 (Ni, 200 c,p ). What is the peculiarity of addition reactions in benzene?
15. Write the reactions of toluene with the indicated reagents : a) 3H 2 (Ni, 200 C, 9806.7 kPa); b) KMnO 4 V H 2 ABOUT; c*) Сl 2 , light; d*) Cl 2 (Fe); e*) CH 3 Cl (AlCl 3 ); e*) CH 3 COCl(AlCl 3 ); g) HNO 3 (H 2 SO 4 ). For reactions marked with an asterisk, state the mechanisms.
16. Write the reactions of nitration of ethylbenzene under the indicated conditions: a) 65% HNO 3 + H 2 SO 4 (conc.); b) 10% HNO 3 , heating, pressure. Bring mechanisms.
17. Compare the ratio of isopropylbenzene to bromine: a) in the presence of AlBr 3 ; b) when illuminated and heated. Give the reactions and their mechanisms.
18. What compounds are formed from ethylbenzene and P -xylene under the action of the indicated oxidizing agents: a) Oh 3 , then H 2 O(Zn); b) KMnO 4 in H 2 ABOUT,t ; VC 2 Cr 2 O 7 in H 2 SO 4 , t ?
19. With the help of what reactions can the following pairs of compounds be distinguished: a) ethylbenzene and m -xylene; b) ethylbenzene and styrene; c) styrene and phenylacetylene; G) O - And P -xylenes?
20. Which compounds are the products of the following reactions:
21. Based on benzene and any other reagents, obtain the following compounds: a) P -tert -butyltoluene; b) ethyl- P - tolyl ketone; c) alylbenzene; G) P - bromobenzoic acid.
22. Name the main compounds formed in the following reactions:
DEFINITION
Benzene(cyclohexatriene - 1,3,5) - an organic substance, the simplest representative of a number of aromatic hydrocarbons.
Formula - C 6 H 6 (structural formula - Fig. 1). Molecular weight - 78, 11.
Rice. 1. Structural and spatial formulas of benzene.
All six carbon atoms in the benzene molecule are in the sp 2 hybrid state. Each carbon atom forms 3σ bonds with two other carbon atoms and one hydrogen atom lying in the same plane. Six carbon atoms form a regular hexagon (σ-skeleton of the benzene molecule). Each carbon atom has one unhybridized p-orbital, which contains one electron. Six p-electrons form a single π-electron cloud (aromatic system), which is depicted as a circle inside a six-membered cycle. The hydrocarbon radical derived from benzene is called C 6 H 5 - - phenyl (Ph-).
Chemical properties of benzene
Benzene is characterized by substitution reactions proceeding according to the electrophilic mechanism:
- halogenation (benzene interacts with chlorine and bromine in the presence of catalysts - anhydrous AlCl 3, FeCl 3, AlBr 3)
C 6 H 6 + Cl 2 \u003d C 6 H 5 -Cl + HCl;
- nitration (benzene easily reacts with a nitrating mixture - a mixture of concentrated nitric and sulfuric acids)
- alkylation with alkenes
C 6 H 6 + CH 2 \u003d CH-CH 3 → C 6 H 5 -CH (CH 3) 2;
Addition reactions to benzene lead to the destruction of the aromatic system and proceed only under harsh conditions:
- hydrogenation (the reaction proceeds when heated, the catalyst is Pt)
- addition of chlorine (occurs under the action of UV radiation with the formation of a solid product - hexachlorocyclohexane (hexachlorane) - C 6 H 6 Cl 6)
Like any organic compound, benzene enters into a combustion reaction with the formation of carbon dioxide and water as reaction products (it burns with a smoky flame):
2C 6 H 6 + 15O 2 → 12CO 2 + 6H 2 O.
Physical properties of benzene
Benzene is a colorless liquid, but has a specific pungent odor. Forms an azeotropic mixture with water, mixes well with ethers, gasoline and various organic solvents. Boiling point - 80.1C, melting point - 5.5C. Toxic, carcinogen (i.e. contributes to the development of cancer).
Obtaining and using benzene
The main methods for obtaining benzene:
— dehydrocyclization of hexane (catalysts - Pt, Cr 3 O 2)
CH 3 -(CH 2) 4 -CH 3 → C 6 H 6 + 4H 2;
- dehydrogenation of cyclohexane (the reaction proceeds when heated, the catalyst is Pt)
C 6 H 12 → C 6 H 6 + 4H 2;
– trimerization of acetylene (the reaction proceeds when heated to 600C, the catalyst is activated carbon)
3HC≡CH → C 6 H 6 .
Benzene serves as a raw material for the production of homologues (ethylbenzene, cumene), cyclohexane, nitrobenzene, chlorobenzene, and other substances. Previously, benzene was used as an additive to gasoline to increase its octane number, however, now, due to its high toxicity, the content of benzene in fuel is strictly regulated. Sometimes benzene is used as a solvent.
Examples of problem solving
EXAMPLE 1
| Exercise | Write down the equations with which you can carry out the following transformations: CH 4 → C 2 H 2 → C 6 H 6 → C 6 H 5 Cl. |
| Solution | To obtain acetylene from methane, the following reaction is used: 2CH 4 → C 2 H 2 + 3H 2 (t = 1400C). Obtaining benzene from acetylene is possible by the reaction of trimerization of acetylene, which occurs when heated (t = 600C) and in the presence of activated carbon: 3C 2 H 2 → C 6 H 6 . The chlorination reaction of benzene to obtain chlorobenzene as a product is carried out in the presence of iron (III) chloride: C 6 H 6 + Cl 2 → C 6 H 5 Cl + HCl. |
EXAMPLE 2
| Exercise | To 39 g of benzene in the presence of iron (III) chloride was added 1 mol of bromine water. What amount of the substance and how many grams of what products did this result in? |
| Solution | Let us write the equation for the reaction of benzene bromination in the presence of iron (III) chloride: C 6 H 6 + Br 2 → C 6 H 5 Br + HBr. The reaction products are bromobenzene and hydrogen bromide. The molar mass of benzene, calculated using the table of chemical elements of D.I. Mendeleev - 78 g/mol. Find the amount of benzene substance: n(C 6 H 6) = m(C 6 H 6) / M(C 6 H 6); n(C 6 H 6) = 39/78 = 0.5 mol. According to the condition of the problem, benzene reacted with 1 mol of bromine. Consequently, benzene is in short supply and further calculations will be made for benzene. According to the reaction equation n (C 6 H 6): n (C 6 H 5 Br) : n (HBr) \u003d 1: 1: 1, therefore n (C 6 H 6) \u003d n (C 6 H 5 Br) \u003d: n (HBr) \u003d 0.5 mol. Then, the masses of bromobenzene and hydrogen bromide will be equal: m(C 6 H 5 Br) = n(C 6 H 5 Br)×M(C 6 H 5 Br); m(HBr) = n(HBr)×M(HBr). Molar masses of bromobenzene and hydrogen bromide, calculated using the table of chemical elements of D.I. Mendeleev - 157 and 81 g/mol, respectively. m(C 6 H 5 Br) = 0.5×157 = 78.5 g; m(HBr) = 0.5 x 81 = 40.5 g. |
| Answer | The reaction products are bromobenzene and hydrogen bromide. The masses of bromobenzene and hydrogen bromide are 78.5 and 40.5 g, respectively. |
Physical properties
Benzene and its closest homologues are colorless liquids with a specific odor. Aromatic hydrocarbons are lighter than water and do not dissolve in it, but they easily dissolve in organic solvents - alcohol, ether, acetone.
Benzene and its homologues are themselves good solvents for many organic substances. All arenas burn with a smoky flame due to the high carbon content of their molecules.
The physical properties of some arenes are presented in the table.
Table. Physical properties of some arenas
|
Name |
Formula |
t°.pl., |
t°.bp., |
|
Benzene |
C 6 H 6 |
5,5 |
80,1 |
|
Toluene (methylbenzene) |
C 6 H 5 CH 3 |
95,0 |
110,6 |
|
Ethylbenzene |
C 6 H 5 C 2 H 5 |
95,0 |
136,2 |
|
Xylene (dimethylbenzene) |
C 6 H 4 (CH 3) 2 |
||
|
ortho- |
25,18 |
144,41 |
|
|
meta- |
47,87 |
139,10 |
|
|
pair- |
13,26 |
138,35 |
|
|
Propylbenzene |
C 6 H 5 (CH 2) 2 CH 3 |
99,0 |
159,20 |
|
Cumene (isopropylbenzene) |
C 6 H 5 CH(CH 3) 2 |
96,0 |
152,39 |
|
Styrene (vinylbenzene) |
C 6 H 5 CH \u003d CH 2 |
30,6 |
145,2 |
Benzene - low-boiling ( tkip= 80.1°C), colorless liquid, insoluble in water
Attention! Benzene - poison, acts on the kidneys, changes the blood formula (with prolonged exposure), can disrupt the structure of chromosomes.
Most aromatic hydrocarbons are life threatening and toxic.
Obtaining arenes (benzene and its homologues)
In the laboratory
1. Fusion of salts of benzoic acid with solid alkalis
C 6 H 5 -COONa + NaOH t → C 6 H 6 + Na 2 CO 3
sodium benzoate
2. Wurtz-Fitting reaction: (here G is halogen)
From 6H 5 -G+2Na + R-G →C 6 H 5 - R + 2 NaG
WITH 6 H 5 -Cl + 2Na + CH 3 -Cl → C 6 H 5 -CH 3 + 2NaCl
In industry
- isolated from oil and coal by fractional distillation, reforming;
- from coal tar and coke oven gas
1. Dehydrocyclization of alkanes with more than 6 carbon atoms:
C 6 H 14 t , kat→C 6 H 6 + 4H 2
2. Trimerization of acetylene(only for benzene) – R. Zelinsky:
3С 2 H2 600°C, Act. coal→C 6 H 6
3. Dehydrogenation cyclohexane and its homologues:
Soviet Academician Nikolai Dmitrievich Zelinsky established that benzene is formed from cyclohexane (dehydrogenation of cycloalkanes
C 6 H 12 t, cat→C 6 H 6 + 3H 2
C 6 H 11 -CH 3 t , kat→C 6 H 5 -CH 3 + 3H 2
methylcyclohexanetoluene
4. Alkylation of benzene(obtaining homologues of benzene) – r Friedel-Crafts.
C 6 H 6 + C 2 H 5 -Cl t, AlCl3→C 6 H 5 -C 2 H 5 + HCl
chloroethane ethylbenzene
Chemical properties of arenes
I. OXIDATION REACTIONS
1. Combustion (smoky flame):
2C 6 H 6 + 15O 2 t→12CO 2 + 6H 2 O + Q
2. Benzene under normal conditions does not decolorize bromine water and an aqueous solution of potassium permanganate
3. Benzene homologues are oxidized by potassium permanganate (discolor potassium permanganate):
A) in an acidic environment to benzoic acid
Under the action of potassium permanganate and other strong oxidants on the homologues of benzene, the side chains are oxidized. No matter how complex the chain of the substituent is, it is destroyed, with the exception of the a -carbon atom, which is oxidized into a carboxyl group.
Homologues of benzene with one side chain give benzoic acid:
Homologues containing two side chains give dibasic acids:
5C 6 H 5 -C 2 H 5 + 12KMnO 4 + 18H 2 SO 4 → 5C 6 H 5 COOH + 5CO 2 + 6K 2 SO 4 + 12MnSO 4 + 28H 2 O
5C 6 H 5 -CH 3 + 6KMnO 4 + 9H 2 SO 4 → 5C 6 H 5 COOH + 3K 2 SO 4 + 6MnSO 4 + 14H 2 O
Simplified :
C 6 H 5 -CH 3 + 3O KMnO4→C 6 H 5 COOH + H 2 O
B) in neutral and slightly alkaline to salts of benzoic acid
C 6 H 5 -CH 3 + 2KMnO 4 → C 6 H 5 COO K + K OH + 2MnO 2 + H 2 O
II. ADDITION REACTIONS (harder than alkenes)
1. Halogenation
C 6 H 6 + 3Cl 2 h ν → C 6 H 6 Cl 6 (hexachlorocyclohexane - hexachloran)
2. Hydrogenation
C 6 H 6 + 3H 2 t , PtorNi→C 6 H 12 (cyclohexane)
3. Polymerization
III. SUBSTITUTION REACTIONS – ionic mechanism (lighter than alkanes)
b) benzene homologues upon irradiation or heating
In terms of chemical properties, alkyl radicals are similar to alkanes. Hydrogen atoms in them are replaced by halogens by a free radical mechanism. Therefore, in the absence of a catalyst, heating or UV irradiation leads to a radical substitution reaction in the side chain. The influence of the benzene ring on alkyl substituents leads to the fact that the hydrogen atom is always replaced at the carbon atom directly bonded to the benzene ring (a-carbon atom).
1) C 6 H 5 -CH 3 + Cl 2 h ν → C 6 H 5 -CH 2 -Cl + HCl
c) benzene homologues in the presence of a catalyst
C 6 H 5 -CH 3 + Cl 2 AlCl 3 → (mixture of orta, pair of derivatives) +HCl
2. Nitration (with nitric acid)
C 6 H 6 + HO-NO 2 t, H2SO4→C 6 H 5 -NO 2 + H 2 O
nitrobenzene - smell almond!
C 6 H 5 -CH 3 + 3HO-NO 2 t, H2SO4→ WITH H 3 -C 6 H 2 (NO 2) 3 + 3H 2 O2,4,6-trinitrotoluene (tol, trotyl)
The use of benzene and its homologues
Benzene C 6 H 6 is a good solvent. Benzene as an additive improves the quality of motor fuel. It serves as a raw material for the production of many aromatic organic compounds - nitrobenzene C 6 H 5 NO 2 (solvent, aniline is obtained from it), chlorobenzene C 6 H 5 Cl, phenol C 6 H 5 OH, styrene, etc.
Toluene C 6 H 5 -CH 3 - a solvent used in the manufacture of dyes, drugs and explosives (trotyl (tol), or 2,4,6-trinitrotoluene TNT).
Xylene C 6 H 4 (CH 3) 2 . Technical xylene is a mixture of three isomers ( ortho-, meta- And pair-xylenes) - is used as a solvent and starting product for the synthesis of many organic compounds.
Isopropylbenzene C 6 H 5 -CH (CH 3) 2 serves to obtain phenol and acetone.
Chlorine derivatives of benzene used for plant protection. Thus, the product of substitution of H atoms in benzene with chlorine atoms is hexachlorobenzene C 6 Cl 6 - a fungicide; it is used for dry seed dressing of wheat and rye against hard smut. The product of the addition of chlorine to benzene is hexachlorocyclohexane (hexachloran) C 6 H 6 Cl 6 - an insecticide; it is used to control harmful insects. These substances refer to pesticides - chemical means of combating microorganisms, plants and animals.
Styrene C 6 H 5 - CH \u003d CH 2 polymerizes very easily, forming polystyrene, and copolymerizing with butadiene - styrene-butadiene rubbers.
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